The use of compressor-based cooling plants that employ multiple machines is the most common method of providing cooling for medium and large commercial and institutional buildings and also for many industrial processes. Centrifugal chillers are usually electric driven, but may also be driven by an engine or other power source. Electric driven water chillers are used extensively in buildings, campuses and district cooling plants to provide chilled water for comfort conditioning. It is estimated by Lawrence Berkeley Laboratories that over 20% of all electric power generated in the U.S. is employed for compression-type cooling applications.
It is common to employ multiple chillers arranged in parallel as shown in FIG. 1. Similarly, the use of multiple, parallel pumps to circulate chilled or heating water in variable flow hydronic systems, and the use of multiple, parallel connected fans to provide variable airflow for comfort conditioning air in buildings is growing. This invention is directed to improved methods of sequencing multiple, parallel connected centrifugal chillers, pumps or fans in systems that employ variable speed operation of the units in such a system. Hereinafter, the term "unit" is used to refer to a chiller, pump or fan as the context may require. The term "sequencing" refers to activating or bringing on-line parallel units in a system, and conversely, deactivating or taking parallel units off-line in an HVAC system.
The present invention will be described and illustrated as applied in the context of centrifugal type water chillers which are the least costly and most efficient type of large chiller. Centrifugal chillers are the most commonly employed refrigeration machine type for larger comfort cooling applications and have been employed for in this role for over 75 years. The invention is not limited to such applications, however, as further explained later. For example, it can be advantageously applied to variable-flow pumping applications where multiple variable-speed drive centrifugal pumps are deployed in parallel, such as for pumping heating or cooling fluids from a header to the loads.
Chilled water plants typically include multiple chillers. This permits staging equipment to meet the changing loads, which usually vary from 0% to 100% of maximum load depending on weather or process requirements. Multiple units also permit fail safe operation with backup available in case of a failure of one of the machines. FIG. 1 illustrates the major components of a conventional water chiller plant with four water-cooled chillers. Chiller plants typically employ two to as many as a dozen or more chillers for comfort conditioning applications or to serve process cooling needs in a manufacturing application. In a typical water chillers plant, each chiller has a motor/compressor unit (109a-d), which may be a hermetic type or open type. The motor or engine drives the corresponding compressor. Each compressor draws low pressure refrigerant gas from the corresponding cooler (11a-d), compresses it, and discharges it as a higher pressure hot gas into the condenser (112a-d). In the condenser, hot gaseous refrigerant is condensed into a liquid by rejecting heat to tepid water from a cooling tower which cools the water by a combination of heat and mass transfer. In the towers, distribution nozzles (122a-d) spread the water over tower fill (123a-d) to enhance cooling by evaporation. A fan in each tower (124a-d), driven by a motor (125a-d), directs air through the water as it cascades through the fill to the tower basin (126a-d). The water is cooled by the evaporation process that occurs as it falls through the tower fill. The cooler water in the tower basins is circulated through the chiller condensers to condense the hot refrigerant and then back to the cooling towers distribution nozzles by condenser water pumps (120a-d).
The condensed liquid refrigerant flows through an expansion device in each chiller (133a-d) that regulates the flow of refrigerant into each cooler (111a-d), which are held at a low pressure by the operation of the compressor. The low pressure environment causes the refrigerant to change state to a gas and as it does so, it absorbs the required heat of vaporization from the chilled water circulating through the cooler due to operation of chilled water pumps (140a-d). The low pressure vapor is drawn into the inlet of the compressor and the cycle is continuously repeated. The chilled water is circulated to a distribution header (181) where it is delivered to the cooling loads served by the plant and is return from those loads into a return header (182) for circulation through the on-line chiller(s) again. Though the configuration of many chilled water plants is similar to that shown in FIG. 1, there are many variations to this basic design.
Because all cooling plants that provide chilled water for comfort conditioning and most plants that provide process cooling are subject to wide variations in cooling load size, some method of modulating the capacity of the chiller plant is necessary. In the prior art, this is accomplished by sequencing equipment based on the load being served such that a minimum amount of equipment operates at all times, sufficient to meet the current load, and the equipment that remains operational, i.e. the units "on-line," are operated at as close to full capacity as possible. Thus, as the load served by the plant increases, additional chillers, along with their associated pumps and fans are started.
It is generally known and accepted in this field that the cooling load should be distributed evenly among the operating chillers. A common strategy for sequencing chillers, in other words, making the determination of when to activate another chiller, or conversely when to deactivate a chiller, is typically accomplished by the "capacity" method in which additional chillers are sequenced on when the operating units have insufficient capacity to meet the current load, and chillers are sequenced off (called "shedding") when the current load can be met with one fewer machines operating. A description of chiller shedding based on the capacity method is disclosed in U.S. Pat. No. 5,222,370. Capacity based sequencing approaches in which chiller capacity and or sequencing is adjusted using current head pressure or condensing temperature are disclosed in U.S. Pat. Nos. 4,210,957 and 4,463,574. Another known approach for chiller sequencing, one specifically for chiller plants that incorporate equipment of different efficiencies, in which chillers staging is adjusted based on capacity and relative efficiency using a predetermined matrix is disclosed in U.S. Pat. No. 4,483,152. Another example of capacity control integrated with capacity-based sequencing for a system employing variable speed chillers is disclosed in U.S. Pat. No. 5,097,670. Although the means to calculate current capacity and required capacity vary, and the selection of chillers to add or shed may be based on efficiency of individual units at that operating point, all of these prior art techniques employ sequencing of equipment on or off line based on the capacity of the various equipment such that the on-line equipment is operated as near as possible to full capacity, and equipment is sequenced off line when the current equipment has sufficient excess capacity such that the load can be met with at least one less piece of equipment on-line. Some find it intuitive that operating each unit at or near its full capacity will lead to efficient operation of the overall system.
Similarly, air and water distribution systems that employ multiple centrifugal pumps and fans are also sequenced on and off line in response to loading (capacity) only. Accordingly, a pump or fan is added when the units currently on-line lack sufficient capacity to meet the current load conditions, and a pump or fan is sequenced off line with the on-line units are determined to have sufficient capacity with one fewer units on-line.
Many manufacturers of chillers now make centrifugal chillers with an option for variable speed drive operation. Nonetheless, the present state of the art is to sequence variable speed chillers using methods that are the same as or very similar to the capacity methods described above, i.e. those that are used in plants that employ conventional constant speed equipment. The use of variable speed drives for condenser pumps is presently discouraged by chiller manufacturers because of the perceived difficulty in controlling the speed of such pumps adequately. However, I describe stable methods for operation of variable flow chilled water cooling systems in my U.S. Pat. No. 5,946,926 incorporated herein by this reference. I believe that systems using variable speed drives for chilled water pumps, condenser pumps and cooling tower fans are likely to become increasingly popular in the future because of various advantages, the most important being the opportunities for improved operating efficiency as described below. Such all-variable speed plants are commonly called "LOOP" chiller plant configurations.
As noted, it is generally assumed that chillers and other refrigeration equipment operate most efficiently when operating at or very near full load conditions. This widely accepted design principle is reinforced by the fact that ancillary equipment such as chilled and/or condenser water pumps in conventional constant speed chiller plants are also constant speed and use the same energy at all chiller loads. This and the fact that centrifugal chiller compressors are typically selected for peak efficiency at design conditions makes it appear that efficiency will be lost if such chillers are operated at loads below their rated maximum capacity. I have re-examined these assumptions and discovered a new strategy for sequencing HVAC equipment that provides substantial improvements in operating efficiencies, notwithstanding that it runs counter to conventional wisdom and perhaps intuition as well.